Peak detection when receiving a signal from a transducer

That sounds like a great project for someone's master's thesis. Maybe you could fund them?

this is my final project for my bach of science

It can be done based on resistence using a digital SWR meter. There should be a resistence to thickness factor based on signal strength.

I think there are a few ways of telling the thickness of coatings of various types on various substraetes.

It seems to be a mature technology that you can buy off the shelf for your application,

http://www.google.ca/search?hl=en&q=%22thickness+gage%22+%2Bcoating&btnG=Google+Search

the material does not conduct electricity and therefore cannot be done by resistance

well, if you use electromagnetic waves you are not going to be able to make it work. Let us say.001 inch paint. SInce paint is transparent you will not get a first surface reflection.

So it must be sound or the usual capacitive means.

So .001 : and you want to detect, say 5% of that or a variation of .00005.

That means that 1/4 wave must be 0.00005 or a full wave is .0002, since sound travels 1000 feet/second is second will hold 1000/.0002 full wavelengths = 5 Megahertz sonar. They usually send a short pulse and look for the reflection, so you want to operate at 5-10 times that frequency or 25-50 Mhz sound waves.

Seems like quite a task. WHat is the highest frequency medical sonogram?

http://en.wikipedia.org/wiki/Ultrasonography

i am actually taking a sine wave and modulating it with a square wave. then i measure the difference in the peak when a maxima occurs due to interference by itself. this was published by pant and skinner in october of '06. like i said it has already been tested and i just want to make it a handheld device.

oops i forgot to log in ^ was me, guest

Just a few alternative ideas.

You would need to send out a signal and receive one. Possible applications may be an IR diode to transmit, and a photo diode to receive the reflected energy. Based on the voltage drop, or change in resistence in the photo diode. This would need to be read on a non painted part for calibration at first. This would need to be done in a black box with no light entering. Another idea would be to get a sensitive enough scale to measure the difference in weight of the painted part from a non painted one. If the paint application is pretty much constant everytime as in a powder coat application. Hopefully you will get the answer you are looking for. I'm here for some ideas. Good luck.